This invention relates to the accurate measurement and monitoring of fine relative positions or displacements, eg. rotational or angular separations or displacements, vibrations, linear separations or translations, alignments and misalignments. Of particular, though not exclusive, interest is measurement of angles.
Known devices designed for ultra precise measurement of angles include autocollimators, diffraction based systems and gears based systems. Autocollimators use measurement of angular deviation to determine in turn, eg, straightness, flatness, squareness and parallelism. Modern forms use laser diode light sources and beamsplitters, and incorporate a micrometer in the eyepiece viewing system for accurate measurement of angular displacement. Typical best accuracies are 0.2 arcseconds, for a measuring range of 150 arcseconds.
In a known goniometer-style instrument, a pair of radial gratings rotate in unison at a uniform speed and are scanned by a pair of reading heads. One of these is stationary while the other moves through the angle to be measured. The relative phase change between the two resultant signals is an indication of the rotation of the moveable reading head with respect to the fixed head. Accuracy achieved is said to be 0.1 arcsecond.
These prior devices are relatively expensive and typically rather large instruments. Often they form a key part of another scientific apparatus, such as a diffractemeter, where the precise measure of angle determines the resolution and quality of an instrument.
Accuracy of angular measurement is the subject of Zhang et al. xe2x80x9cImproving the Accuracy of Angle Measurement System with Optical Gratingxe2x80x9d, Annals of the CIRP Vol 43, No. 1 (1994). This paper proposes the use of index gratings with sine function transmissivity, and other enhancements, and reports an accuracy of 0.2 arcseconds with a prototype instrument.
It is an object of this invention to provide for fine measurement and monitoring of relative positions or displacements, whether angular, linear or otherwise, to a satisfactory accuracy that is preferably better than that achieved with known instruments and techniques.
The invention proposes an approach quite different from that previously used, and entails monitoring the quantum tunnelling current between two proximate electrical conductors, preferably of nano dimensions. In a preferred embodiment, two arrays of aligned conductors may be used, and these may advantageously be carbon nanotubes.
The invention accordingly provides, in a first aspect, a method of measuring and/or monitoring the relative position or displacement of two elements, including:
associating the elements with respective elongate electrical conductors;
disposing the conductors preferably in approximate alignment, at a mutual separation and applying an electrical potential difference such that there is a detectable quantum tunnelling current between them; and
detecting and/or measuring said quantum tunnelling current.
Preferably, the relative positions of the conductors is adjusted to determine that position at which maximum quantum tunnelling current is detected.
In a second aspect, the invention provides apparatus for use in measuring and/or monitoring the relative position or displacement of two elements. The apparatus includes a pair of elongate electrical conductors adapted to be associated with the respective elements, and means for disposing the conductors, preferably substantially aligned in mutually parallel relationship, at a mutual separation such that a detectable quantum tunnelling current may be generated between them on the application of an electrical potential difference between the conductors.
The apparatus may further include means to apply said potential difference, and means to detect and/or measure the quantum tunnelling current between the conductors.
Preferably, the apparatus further includes means to adjust the relative positions of the conductors to determine that position at which maximum quantum tunnelling current is detected.
The position or displacement may be one or more of a rotational or angular separation or displacement, a vibration, a linear separation or translation, an alignment and a misalignment.
Preferably, the electrical conductors are of width 1 micron or less eg, in one or more embodiments, of width in the nano-order to sub-micron range. In the latter case, the conductors may be carbon nanotubes of arbitrary helicity or radius, either single or multi-walls of carbon monofilaments, or nanowires. Alternatively, the conductors may be, eg, micron to sub-micron quasi one-dimensional conductors. In some embodiments, the conductors may be of length 1 mm or less.
The conductors may be associated with the aforesaid elements by being mounted in or on an insulating or semiconducting substrate, preferably flush with a surface of the substrate. The substrate may be, eg. a solid or a crystal face. The conductors may be placed along respective atomic steps on a vicinal surface.
Advantageously, the electrical conductors are arranged in respective ordered grids or arrays of electrical conductor segments, preferably wired in parallel e.g. through a single supply lead, which grids or arrays are complementary and overlaid to place the conductor segments in sufficient proximity to obtain detectable quantum tunnelling currents.